1. Field of the Invention
The present invention relates to a hydraulic shock absorber for a vehicle, such as a front fork, a rear damper or the like of a motorcycle or the like.
2. Description of the Related Art
In a front fork of a motorcycle, there have been structures in which wear resistance and low-frictional resistance are improved by forming a hard film of TiN or TiO compound on a surface of a slide pipe slidably provided within a cylinder tube in accordance with a dry plating such as an HCD method, a PVD method, a PVCD method or the like. The structure is used in a road racer, an off road racer, a mass production motorcycle or the like. In this case, a surface of the TiN film has a gold color, and a surface of the TiO film has a blue color, a green color or a brown color in correspondence to an amount of contained oxygen.
Since the front fork of the motorcycle is mounted to the body with a caster angle, a side force applied from a road surface is transmitted in the front fork. This side force is applied to a guide bush provided in an inner periphery of an opening portion in a cylinder tube of the front fork, and a guide bush provided in an outer periphery of a leading end portion in the slide pipe.
When the motorcycle travels on a flat straight road surface of the like, vibration input from the road surface is small, so that only a small side force is applied to the slide pipe. Further, when traveling on the flat straight road surface or the like, the slide pipe is driven with respect to the cylinder tube at a low speed.
On the contrary, when the motorcycle travels on a rough road surface or the like having great irregularity, substantial vibration is input from the road surface, so that a great side force is applied to the slide pipe. Further, when traveling on a rough road surface or the like having great irregularity, the slide pipe is driven at a high speed.
However, in the conventional slide pipe on which the TiN or the TiO is coated, since frictional resistance with respect to a guide bush and an oil seal fixed to the opening portion of the cylinder tube is small, when the front fork is driven at a low speed such as when traveling on a flat straight road surface or the like, wasteful motions may be generated in the slide pipe and the road holding property of the tire is deteriorated. On the contrary, when passing through a rough road surface or the like having great irregularity, the friction may be great, and the motion of the slide pipe is slow, that is, a road surface following property of the tire is deteriorated.
In this case, recently, there has been disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-214269, a technique of coating an amorphous hard carbon film (called as DLC (Diamond Like Carbon) film) on a metal base plate of a sliding part used in an automobile or the like. However, in the prior art, there is nothing which suggests the problems mentioned above with regard to the motion of the slide pipe peculiar to the hydraulic shock absorber for a vehicle such as the front fork or the like, and there is disclosed nothing relating to forming the DLC film on the slide member of the hydraulic shock absorber for a vehicle.